Currently smart power semiconductor switches are used in a broad variety of applications. Not only in automotive applications smart semiconductor switches are increasingly used to replace electro-mechanical relays. Particularly in battery powered systems (e.g., the electronic equipment of a car) a sufficient reverse polarity protection is a requirement. In a typical automotive application the nominal supply voltage is +12 V. A robust electronic device is usually required to withstand a supply voltage up to −16 V (reverse voltage) for at least two minutes.
A multi-channel switching device usually includes one power semiconductor switch (usually a MOSFET) per output channel wherein an electric load is connected to each output channel. Thus each electric load may be switched on and off using the respective semiconductor switch. Assuming an on-resistance of 100 mΩ and a nominal load current of 1 A results in a power loss of 100 mW for each active output channel. Each power semiconductor switch have usually a reverse diode coupled in parallel to the load current path of the semiconductor switch (e.g., the drain-source path in case of a MOSFET). During normal operation this reverse diode is reverse biased and in a blocking state. However, when a negative supply voltage is applied, the reverse diode becomes forward biased and load current may be directed form ground through the load and the reverse diode to the negative supply potential. Assuming a diode forward voltage of at least 0.7 V the resulting power loss in the reverse diode (and thus in the switching device) is 700 mW per output channel, at least seven times more as during normal operation. It goes without saying that such a situation may be hazardous for the switching device and appropriate reverse polarity protection circuitry is required.
Known circuitry providing reverse polarity protection to smart semiconductor switches is comparably complex and requires significant chip space. Thus there is a need for a smart semiconductor switch including an efficient (as far as circuit complexity and chip space requirement are concerned) reverse polarity protection.